In both types of preheater a hot medium such as an exhaust gas from a boiler, is brought first into contact with a given portion of the regenerative mass and gives up heat to it; then, a cold medium to be heated, such as air, is brought into contact with that portion of the mass and takes up heat from it.
Attempts have been made to maximise the utilisation of heat from the hot medium, in relation to the cold medium to be heated, particularly from the point of view of reducing the final temperature at which the erstwhile hot medium is discharged to the surrounding atmosphere.
One manner in which this has been attempted in the past is seen from German Pat. No. 1,264,672 wherein preheating elements and economizer heating elements are arranged alternately in series, each preheater region being succeeded by one or more heat exchange regions in which preheater and economizer elements or alternatively preheater and superheater elements are disposed in juxtaposition. This amounts to a stepped arrangement of a plurality of heat exchanger regions and this results in a considerable increase in the overall volume of the installation.